1. Field of Invention
The present invention relates to a synchronous driver circuit, and particularly it relates to a synchronous driver circuit with a bootstrap capacitor; for example, it can be used to drive light emitting diodes (LEDs). The present invention also relates to an integrated circuit for use therein.
2. Description of Related Art
Referring to FIG. 1, a power regulator 100 is often required for supplying power to a load circuit 200. In certain applications, the input voltage Vin and the output voltage Vout of the power regulator 100 do not always have a consistent relationship. For example, the same power regulator 100 may be employed in different applications which require different output voltages Vout for different load circuits 200. Under such circumstance, the input voltage Vin may be higher or lower than the output voltage Vout; hence, the power regulator 100 should be designed in such a way that it can meet both conditions. As another example, when the input voltage Vin is supplied by a battery, the input voltage Vin is higher than the output voltage Vout in the beginning; yet, as time passes, the battery voltage decreases, and the input voltage Vin gradually becomes lower than the output voltage Vout. In order to regulate the output voltage under the circumstance that the input voltage Vin and the output voltage Vout of the power regulator 100 do not have a consistent relationship, U.S. Pat. No. 6,788,033 discloses a synchronous buck-boost power regulator. As shown in FIG. 2, in this prior art, two sets of synchronous switches (i.e., four power transistors: Q11 and Q12, Q21 and Q22) are employed to perform buck conversion when the input voltage is higher than the output voltage, and boost conversion when the output voltage is higher than the input voltage.
In an application to drive LEDs, as shown in FIG. 3, U.S. Pat. No. 5,739,639 discloses an asynchronous structure wherein an inductor L, a transistor Q, and a diode D constitute an asynchronous boost power regulator. This prior art connects the LED circuit 200 between the output voltage Vout and the input voltage Vin, not between the output voltage Vout and ground. As such, the operational voltage (load voltage) of the LED circuit 200 becomes (Vout-Vin); this operational voltage can be higher or lower than the input voltage Vin. However, regardless whether the operational voltage is higher or lower than the input voltage Vin, proper power conversion can be achieved by a boost type power regulator.
The foregoing prior art circuits have respective drawbacks. U.S. Pat. No. 6,788,033 requires four power transistors and more complicated feedback control mechanism. U.S. Pat. No. 5,739,639 employs asynchronous structure, and it is well known that the power conversion efficiency of such structure is inferior.
In view of the aforementioned drawbacks, the present invention provides a synchronous driver circuit. In a synchronous driver circuit, facing a requirement that the input voltage and the load voltage do not have a consistent relationship, there may not be sufficient driving strength to drive the up-gate power transistor because the load circuit requires a higher voltage. The present invention provides a solution to this issue.